This invention relates to redundant driving mechanism, i.e. to an actuator inherently having unusually high assurance of performance. Redundant drive arrangements are somtimes required or found desireable in critical operating situations on earth, and are depended upon, perhaps to an even greater degree, in the case of functions to be performed in adverse environments, for instance in outer-space. Specifically, a reduntant acuator is one which may reliably be expected to perform in at least one of two alternative modes, one of which is usually relied on for regular operation and the other constituting a standby, i.e. one which primarily functions in an emergency such as failure in the first mode.
In many instances it is important in a redundant drive arrangement to be able to transmit power with a compact assembly that is not unusually heavy or complex despite the presence of some ordinarily superfluous mechanism. The present invention accordingly takes note of the advantages residing in harmonic drive planetary gearing as basically disclosed, for example, in U.S. Pat. No. 2,906,143. More especially the present invention advantageously incorporates in novel combination the space-saving and speed-changing features embodied in a so-called "Dual Strain Wave Gearing" of the single stage type disclosed in U.S. Pat. No. 2,943,513, issued July 5, 1960, on an application filed in the name of C. W. Musser.